The present invention relates to a radio transmitter. In particular, it relates to a transmitter architecture for cellular mobile phones. Such an architecture has a phased lock loop (PLL).
There are traditionally two types of transmitter, on-channel and offset.
FIG. 1 of the accompanying drawings illustrates an on-channel transmitter. This transmitter comprises a PLL 10, a reference frequency divider 11, a transmitter buffer 12, a power amplifier (PA) 13 and a modulator 14. The PLL 10 consists of a phase detector 15, a low pass filter (LPF) 16, a voltage controlled oscillator (VCO) 17 and a frequency divider 18. The bandwidth of the PLL 10 is proportional to the bandwidth of the LPF 16 and is a compromise between being small enough to eliminate noise and yet sufficiently large for the loop to have a fast enough response to perturbations.
In operation, the PLL acts as follows. The phase detector multiplies the frequency divided incoming (reference) signal f.sub.ref /R by the frequency divided signal output from the VCO 17, f.sub.uhf /N. Typically, the phase detector 15 generates a pulse width modulated signal, which is either a current or voltage signal, and which is proportional to the phase difference between the two input signals, f.sub.ref /R and f.sub.uhf /N. The resultant voltage is filtered by the low pass filter 16 to generate a DC voltage, the amplitude of which governs the frequency of operation of the VCO 17. Modulation is applied by a modulator 14 either by summing the voltage at the output of the filter (solid line in FIG. 1) or by having a separate modulation control to the VCO 17 (dotted line in FIG. 1). The loop ideally keeps the phase difference between the aforementioned signals f.sub.ref /R and f.sub.uhf /N to zero. Accordingly, the VCO 17 outputs a phase locked signal f.sub.uhf.
In the transmitter of FIG. 1, the output of the PLL is ramped up by the transmitter buffer 12 and then amplified by the PA 13. However, because the VCO frequency equals the transmit frequency, the PLL is affected by a
kick which occurs when the PA 13 operates. The output of the VCO 17, namely f.sub.uhf, is affected. This causes an adjustment in the output of the phase detector 15 to indicate a phase difference between f.sub.ref /R and f.sub.uhf /N, which, in turn, alters the output of the VCO 17. Consequently, a settling time is incurred in reducing the phase difference to zero.
The smaller the bandwidth of the PLL, the greater the settling time. The low loop requirements for analogue systems such as AMPS (advanced mobile phone system), NAMPS (narrowband AMPS), and TACS (total access communication system) results in an unacceptable settling time. For example, the specification for AMPS requires the transmit frequency to reach a final frequency within 2 ms of the PA operating. The PLL does not react quickly enough to PA kick to achieve this maximum 2 ms settling time.
The effect of PA kick, and also radiated transmit kick, is reduced by including large amounts of load insulation (buffer 12).
However, the problem is generally overcome by utilising an offset transmitter in such narrow band systems.
FIG. 2 of the accompanying drawings illustrates an offset transmitter. This transmitter comprises a phase locked loop (PLL) 20, a reference frequency divider 21, a modulator 24, a mixer 19, transmitter buffers 22a and 22b, a band pass filter (BPF) 29 and a power amplifier (PA) 23. The PLL 20 has the same components as the on channel PLL10 and operates as described above. That is, it consists of a phase detector 25, a low pass filter (LPF) 26, a voltage controlled oscillator (VCO) 27 and a frequency divider 28. However, the VCO operates at VHF and is therefore less susceptible to
kick. Because the disturbance is less, the fact that the settling time is extended is of less consequence and accordingly, the transmitter can modulate to low frequencies, such as those utilised in NAMPS.
In the offset transmitter of FIG. 2, the output of the PLL 20, f.sub.rf, is mixed with a low frequency signal f.sub.lo to obtain an intermediate frequency f.sub.if .multidot.f.sub.if is f.sub.rf +f.sub.lo and f.sub.rf -f.sub.lo. Since this signal is typically a low output, more gain stages (transmitter buffers 22a and 22b) are required than in the on channel system. Also, these buffers are needed, as in the on-channel transmitter, to reduce the effects of any frequency kicks. Further, the signal needs to be "cleaned" prior to amplification by the PA 23. This is achieved by a band pass filter 29, such as a SAW filter. These additional components are expensive and take up a considerable amount of space on a PCB.